Liquid crystal polymers

ABSTRACT

Compounds of formula ##STR1## are provided which may be used in a variety of devices including liquid crystal devices, piezoelectric devices, pyroelectric devices and in optical recording media, wherein X 1  and X 2  are independently selected from H, CN, COCH 3 , CO 2  CH 3 , m=at least 5; X 3  and X 4  are independently selected from formula (IA), ##STR2## wherein Y is selected from COO, OCO, O, S, CHOH, CHF, CH 2  ; Q=(CH 2 ) n  wherein one or more non-adjacent methylenes may be replaced by O and n=1-20; Z is selected from O, S, single covalent bond, COO, OCO; when Y is CH2 then n may also be O; formula (A) ##STR3## represents any mesogenic group; X3 and X 4  are also independently selected from H, OH, OCOR 1 , COOH, CO 2  R 1  (CH 2 ) p  OH, (CH 2 ) p  CO 2  H, --(CH 2 ) p  OR 1  or --(CH 2 ) p  CO 2  R 1  and p=1-20, R 1  =H or C 1=16  alkyl, when R 1  =C 2-16  alkyl the terminal Ch 3  group may be replaced by Br or Cl; provided that at least one of X3 and X4 is selected from formula (IA).

This invention concerns novel liquid crystal polymers (LCP) materials,novel intermediates and methods for preparing them.

BACKGROUND OF THE INVENTION

Liquid crystals can exist in various phases. In essence there are threedifferent classes of liquid crystalline material, each possessing acharacteristic molecular arrangement. These classes are nematic, chiralnematic (cholesteric) and smectic. A wide range of smectic phasesexists, for example smectic A and smectic C. Some liquid crystalmaterials possess a number of liquid crystal phases on varying thetemperature, others have just one phase. For example, a liquid crystalmaterial may show the following phases on being cooled from theisotropic phase:- isotropic - nematic - smectic A - smectic C - solid Ifa material is described as being smectic A then it means that thematerial possesses a smectic A phase over a useful working temperaturerange.

Materials possessing a smectic A (S_(A) phase may exhibit anelectroclinic effect The electroclinic effect was first described by S.Garoff and R. Meyer, Phys. Rev. Lett., 38, 848 (1977). An electroclinicdevice has also been described in UK patent application GB-2 244 566 A.This particular device helps to overcome the poor alignment problems ofelectroclinic (EC) devices using a surface alignment that gives asurface tilt within a small range of angles.

When a smectic A phase is composed of chiral molecules, it may exhibitan electroclinic effect, i.e. a direct coupling of molecular tilt toapplied field The origin of the electroclinic effect in a smectic Aphase composed of chiral polar molecules has been described by Garoffand Meyer as follows. The application of an electric field parallel tothe smectic layers of such a smectic A phase biases the free rotation ofthe transverse molecular dipoles and therefore produces a non-zeroaverage of the transverse component of the molecular polarisation. Whensuch a dipole moment is present and coupled to the molecular chirality,a tilt of the long molecular axis (the director) is induced in a planeperpendicular to the dipole moment.

In thin samples, for example 1-3 mm, and with the smectic layers tiltedor perpendicular with respect to the glass plates the electrocliniceffect is detectable at low applied fields.

In an aligned smectic A sample a tilt of the director is directlyrelated to a tilt of the optic axis. The electroclinic effect results ina linear electro-optic response. The electro-optic effect can manifestitself as a modulation of the effective birefringence of the device.

Electroclinic (EC) devices are useful, for example, in spatial lightmodulators having an output that varies linearly with applied voltage. Afurther advantage of EC devices is that they have high speed responsetimes, much faster than twisted nematic type devices. One known type offerroelectric device is bistable, in contrast the EC device is notbistable and has an output that varies linearly with applied voltage.

The electroclinic effect is sometimes referred to as the soft-modeeffect see G. Andersson et al in Appl. Phys. Lett., 51, 9, (1987).

In general terms, regarding the electroclinic effect, it is advantageousif on applying a small voltage there results a large induced tilt. Anincrease in induced tilt may result in an increase in contrast ratio. Itis also advantageous if a large induced tilt can be obtained at as low avoltage as possible.

It is also advantageous if the relationship between molecular inducedtilt and applied voltage is temperature independent. When an increase inapplied voltage results in little or no change in induced tilt then thematerial being tested is generally referred to as exhibiting asaturation voltage effect.

By S_(A) * is meant a S_(A) phase which contains some proportion ofchiral molecules.

Cholesteric or chiral nematic liquid crystals possess a twisted helicalstructure which is capable of responding to a temperate change through achange in the helical pitch length. Therefore as the temperature ischanged, then the wavelength of the light reflected from the planarcholesteric structure will change and if the reflected light covers thevisible range then distinct changes in colour occur as the temperaturevaries. This means that there are many possible applications includingthe areas of thermography and thermooptics.

The cholesteric mesophase differs from the nematic phase in that in thecholesteric phase the director is not constant in space but undergoes ahelical distortion. The pitch length for the helix is a measure of thedistance for the director to turn through 360°.

By definition, a cholesteric material is chiral material. Cholestericmaterials may also be used in electro-optical displays as dopants, forexample in twisted nematic displays where they may be used to removereverse twist defect. They may also be used in cholesteric to nematicdyed phase change displays where they may be used to enhance contrast bypreventing wave-guiding.

Thermochromic applications of cholesteric liquid crystal materialsusually use thin film preparations of the materials which are thenviewed against a black background These temperature sensing devices maybe placed into a number of applications involving thermometry, medicalthermography, non-destructive testing, radiation sensing and fordecorative purposes. Examples of these may be found in D. G. McDonnellin Thermotropic Liquid Crystals, Critical Reports on Applied Chemistry,Vol. 22, edited by G. W. Gray, 1987 pp 120-44; this reference alsocontains a general description of thermochromic cholestric liquidcrystals.

Generally, commercial thermochromic applications require the formulationof mixtures which possess low melting points, short pitch lengths andsmectic transitions just below the required temperature-sensing region.Preferably the mixture or material should retain a low melting point andhigh smectic - cholesteric transition temperatures.

In general, thermochromic liquid crystal devices have a thin film ofcholesterogen sandwiched between a transparent supporting substrate anda black absorbing layer. One of the fabrication methods involvesproducing an `ink` with the liquid crystal by encapsulating it in apolymer and using printing technologies to apply it to the supportingsubstrate. Methods of manufacturing the inks include gelatinmicroencapsulation, U.S. Pat. No. 3,585,318 and polymer dispersion, U.S.Pat. Nos. 1,161,039 and 3,872,050. One of the ways for preparingwell-aligned thin-film structures of cholesteric liquid crystalsinvolves laminating the liquid crystal between two embossed plasticsheets. This technique is described in UK patent 2,143,323.

Ferroelectric smectic liquid crystal materials, which can be produced bymixing an achiral host and a chiral dopant, use the ferroelectricproperties of the tilted chiral smectic C, F, G, H, I, J and K phases.The chiral smectic C phase is denoted S_(C) * with the asterisk denotingchirality. The S_(C) phase is generally considered to be the most usefulas it is the least viscous. Ferroelectric smectic liquid crystalmaterials should ideally possess the following characteristics: lowviscosity, controllable spontaneous polarisation (Ps) and an S_(C) phasethat persists over a broad temperature range which should includeambient temperature and exhibits chemical and photochemical stability.Materials which possess these characteristics offer the prospect of veryfast switching liquid crystal containing devices. Some applications offerroelectric liquid crystals are described by J. S. Patel and J. W.Goodby in Opt Eng., 1987, 26,273.

In ferroelectric liquid crystal devices the molecules switch betweendifferent alignment directions depending on the polarity of an appliedelectric field. These devices can be arranged to exhibit bistabilitywhere the molecules tend to remain in one of two states until switchedto the other switched state. Such devices are termed surface stabilisedferoelectric devices, e.g. as described in U.S. Pat. No. 5,061,047 andU.S. Pat. No. 4,367,924 and U.S. Pat. No. 4,563,059. This bistabilityallows the multiplex addressing of quite large and complex devices.

One common multiplex display has display elements, i.e. pixels, arrangedin an X, Y matrix format for the display of for example alpha numericcharacters. The matrix format is provided by forming the electrodes onone slide as a series of column electrodes, and the electrodes on theother slide as a series of row electrodes. The intersections betweeneach column and row form addressable elements or pixels. Other matrixlayouts are known, e.g. seven bar numeric displays.

There are many different multiplex addressing schemes. A common featureinvolves the application of a voltage, called a strobe voltage to eachrow or line in sequence. Coincidentally with the strobe applied at eachrow, appropriate voltages, called data voltages, are applied to allcolumn electrodes. The differences between the different schemes lies inthe shape of the strobe and data voltage waveforms.

Other addressing schemes are described in GB-2,146,473-A;GB-2,173,336-A; GB-2,173,337-A; GB-2, 173629-A; WO 89/05025; Harada etal 1985 S.I.D. Paper 8.4 pp 131-134; Lagerwall et al 1985 I.D.RC. pp213-221 and P Maltese et al in Proc 1988 I.D.R.C. pp 90-101 FastAddressing for Ferroelectric LC Display Panels.

The material may be switched between its two states by two strobe pulsesof opposite sign, in conjunction with a data waveform. Alternatively, ablanking pulse may be used to switch the material into one of itsstates. Periodically the sign of the blanking and the strobe pulses maybe alternated to maintain a net d.c. value.

These blanking pulses are normally greater in amplitude and length ofapplication than the strobe pulses so that the material switchesirrespective of which of the two data waveforms is applied to any oneintersection. Blanking pulses may be applied on a line by line basisahead of the strobe, or the whole display may be blanked at one time, ora group of lines may be simultaneously blanked.

It is well known in the field of ferroelectric liquid crystal devicetechnology that in order to achieve the highest performance fromdevices, it is important to use mixtures of compounds which givematerials possessing the most suitable ferroelectric smecticcharacteristics for particular type of devices.

Devices can be assessed for speed by consideration of the response timevs pulse voltage curve. This relationship may show a minimum in theswitching time (t_(min)) at a particular applied voltage (V_(min)). Atvoltages higher or lower than V_(min) the switching time is longer thant_(min). It is well understood that devices having such a minimum intheir response time vs voltage curve can be multiplex driven at highduty ratio with higher contrast than other ferroelectric liquid crystaldevices. It is preferred that the said minimum in the response time vsvoltage curve should occur at low applied voltage and at short pulselength respectively to allow the device to be driven using a low voltagesource and fast frame address refresh rate.

Typical known materials (where materials are a mixture of compoundshaving suitable liquid crystal characteristics) which do not allow sucha minimum when included in a ferroelectric device include thecommercially available materials known as SCE13 and ZLI-3654 (bothsupplied by Merck UK Ltd, Poole, Dorset). A device which does show sucha minimum may be constructed according to PCT GB 88/01004 and utilisingmaterials such as e.g. commercially available SCE8 (Merck UK Ltd). Otherexamples of prior art materials are exemplified by PCT/GB 86/00040, PCTGB 87/00441 and UK 2232416B.

The unit that is the basic building block of a polymer is called amonomer.

The polymerisation process i.e. the formation of a polymer from itsconstituent monomers does not usually create polymers of uniformmolecular weight, rather what is created is a distribution of molecularweights. In order to describe a sample of polymer it is necessary tostate the average number of monomers in a polymer this is called thedegree of polymerisation (D.P). By how much the majority of polymermolecules differ from this average value (or to describe the spread ofmolecular weight) is called the polydispersity.

A number of different average molecular weights can be drawn from gelpermeation chromatography (GPC) for a given sample including: M_(n)--number average molecular weight and M_(w) --weight average molecularweight. The value used to calculate D.P. is usually M_(n) andpolydispersity is usually defined as M_(w) /M_(n).

Polymers can be made from different types of monomers, in which case thepolymer is called a co-polymer. If two types of monomer join in a randomfashion then the polymer is called a random co-polymer. If the twomonomers form short sequences of one type first which then combine toform the final polymer then a block copolymer results. If shortsequences of one of the monomers attach themselves as side chains tolong sequences consisting of the other type of monomer then the polymeris referred to as a graft copolymer.

In liquid crystal (LC) polymers the monomers can be attached together inessentially two ways. The liquid crystal part or mesogenic unit of thepolymer may be part of the polymer backbone resulting in a main chain LCpolymer. Alternatively, the mesogenic unit may be attached to thepolymer backbone as a pendant group i.e. extending away from the polymerbackbone; this results in a side-chain LC polymer. These different typesof polymer liquid crystal are represented schematically below. Themesogenic units are depicted by the rectangles. ##STR4##

The side chain liquid crystal polymer can generally be thought of ascontaining a flexible polymer with rigid segments (the mesogenic unit)attached along its length by short flexible (or rigid) units as depictedin the schematic representation below. It is the anisotropic, rigidsection of the mesogenic units that display orientational order in theliquid crystal phases. In order to affect the phases exhibited by theliquid crystal and the subsequent optical properties there are manyfeatures which can be altered, some of these features are particularlypertinent to side-chain liquid crystal polymers. One of these featuresis the flexible part that joins the mesogenic unit to the polymerbackbone which is generally referred to as the spacer group. The lengthand flexibility of this spacer group can be altered. ##STR5##

A number of side-chain liquid crystal polymers are known, for examplesee GB 2146787 A.

Liquid crystal polyacrylates are a known class of liquid crystal polymer(LCP). LCPs are known and used in electro-optic applications, forexample in pyroelectric devices, non-linear optical devices and opticalstorage devices. For example see GB 2146787 and Makromol. Chem. (1985)186 2639-47.

Side-chain liquid crystal polyacrylates are described in PolymerCommunications (1988), 24, 364-365 e.g. of formula: ##STR6## where(CH₂)_(m) is the flexible spacer group and X is the side-chain mesogenicunit and R is hydrogen or alkyl.

Side-chain liquid crystal polychloroacrylates are described in Makromol.Chem. Rapid Commun. (1984), 5,393-398 e.g. of formula: ##STR7## where Ris chlorine.

Patent Application PCT GB 94/00662 describes amongst other things theuse of the Baylis-Hillman Reaction to make a range of novel liquidcrystal polymers.

A method for the preparation of polyacrylate homo- or co-polymers havingthe following repeat unit is described in UK patent application GB9203730.8 ##STR8##

R₁ and R₂ are independently alkyl or hydrogen, R₃ is alkyl, hydrogen orchlorine, m is O or an integer 1-20, W is a linkage group COO or OOC orO and X is a mesogenic group.

One of the main problems with liquid crystal polymers is that they areextremely difficult to align in devices. Essentially there are twotechniques which have been used for aligning liquid crystal polymers. Itis possible to try to align the liquid crystal polymer in a similarmanner as a low molar mass liquid crystal, which is described in moredetail below. Alternatively, mechanical techniques can be used such asshearing. Typically mechanical shearing is performed over hot rollers,this technique is generally only suitable for flexible substrates. It ispossible to shear a sample between glass slides however the glass slidescannot be sealed in the conventional manner.

Materials and Assembling Process of LCDs by Morozumi in Liquid CrystalsApplications and uses, vol. 1 Ed. Bahadur, World Scientific PublishingCo, Pte. Ltd, 1990 pp 171-194 and references therein as the titlesuggests discusses methods for assembling liquid crystal devices.

The technique for aligning low molar mass liquid crystals is typicallyas follows. Transparent electrodes are fabricated on the surfaces of thesubstrates, the substrates typically being made of glass e.g. glassslides. In twisted nematic or super twisted nematic devices, forexample, an alignment process is necessary for both substrates. A thinalignment layer is deposited to align the liquid crystal molecules,typically either organic or inorganic aligning layers are used, forexample SiO deposited by evaporation is a typical inorganic alignmentlayer. One method to form the alignment layer involves rubbing thesurface by textures or cloths. Polyimides have also been employed forthe surface alignment of layers. Polyimide is coated onto the substratesbearing electrodes by a spinner and then cured to form a layer ofapproximately 50 nm thickness. Then each layer surface is repeatedlyrubbed in substantially one direction with an appropriate material. Ifthe liquid crystal molecules are deposited on this layer they areautomatically aligned in the direction made by the rubbing. It is oftenpreferable if the molecules possess a small angle pre-tilt typically2-3°. Higher pre-tilts are sometimes required.

The two substrates are then fixed together for example by adhesive andare kept separate by spacing materials. This results in uniform andaccurate cell spacing. A typical adhesive is an epoxy resin. Thissealing material is usually then precured. The electrodes may then beprecisely aligned for example to form display pixels. The cell is thencured at, for example 100-150° C. At this point the empty liquid crystalcell is complete.

It is at this point that the cell is filled with the liquid crystalmaterial. The opening size in the sealing area of the liquid crystalcell is rather small therefore the cell can be evacuated, for example ina vacuum chamber, and the liquid crystal material forced into the cellvia gas pressure. More than one hole in the sealing area may be used.The empty cell is put into a vacuum chamber and then the vacuum chamberis pumped down. After the cell has been evacuated the open region of thesealant is dipped into the liquid crystal material and the vacuumchamber is brought back to normal pressure. Liquid crystal material isdrawn into the cell as a result of capillary action external gases canbe applied to increase the pressure. When the filling process iscomplete the hole or holes in the sealant is/are capped and the cell iscured at a temperature above the liquid crystal material clearing pointto make the liquid crystal molecular alignment stable and harden thecapping material.

Polymer liquid crystal molecules tend to be more viscous than lowmolecular weight liquid crystal materials and are therefore moredifficult to align and more difficult to fill into devices. Only liquidcrystal polymers with low molecular weights can be flow filled into acell, and once a degree of polymerisation greater than around 30 or 40repeat units is reached, most liquid crystal polymers become so viscousthat flow filling cells is extremely difficult Much slower cooling isneeded in order to try and align liquid crystal polymers and thisusually results in poor uniformity of alignment.

Poorly aligned liquid crystal molecules do not result in the fastswitching high contrast materials and devices that are generallyrequired.

The above techniques are suitable for many liquid crystal materialsincluding those devices which use liquid crystal materials which exhibitand utilise the smectic mesophase e.g. ferroelectrics. Suitablealignment techniques may also be found in GB 2210469 B.

Devices containing ferroelectric liquid crystal mixtures can exhibitfast switching times (faster than 100 μs), Clark and Lagerwall, Appl.Phys. Lett, 36, 89,1980. They can be bistable which means that they canbe multiplexed at high levels using a line-at-a-time scan technique.Ferroelectric materials continue to receive a large amount ofinvestigative attention due to their application in high resolution flatpanel displays. An important feature of devices containing liquidcrystalline materials is that they should exhibit a fast response time.The response time is dependent on a number of factors, one of thesebeing the spontaneous polarisation, denoted Ps (measured in nC cm⁻²). Byadding a chiral dopant to the liquid crystalline mixture the value of Pscan be increased, thus decreasing the response time of the device.Ferroelectric smectic liquid crystal materials, which can be produced bymixing an achiral host and a chiral dopant, use the ferroelectricproperties of the tilted chiral smectic C, F, G, H, I, J, and K phases.The chiral smectic C phase is denoted S_(C) * with the asterisk denotingchirality. The S_(C) * phase is generally considered to be the mostuseful as it is the fastest switching. It is desirable that the materialshould exhibit a long pitch chiral nematic (denoted N*) and S_(A) phaseat temperatures above the chiral smectic phase in order to assistsurface alignment in a device containing liquid crystalline material.Ferroelectric smectic liquid materials should ideally possess thefollowing characteristics: low viscosity controllable Ps and an S_(C) *phase that persists over a broad temperature range, which should includeambient temperature, and exhibits chemical and photochemical stability.Materials which possess these characteristics offer the prospect of veryfast switching liquid crystal containing devices.

Ferroelectric LCDs by Dijon in Liquid Crystals Applications and Uses,vol. 1 Ed. Bahadur, World Scientific Publishing Co. Pte. Ltd, 1990 pp350-360 and references therein discusses alignment processes for smecticphases for low molar mass materials. The filling of cells is believed tobe possible only in the isotropic or nematic phase due to the viscosityof smectic phases. Generally materials with the following phase sequencegive good alignment:

    I--N*--S.sub.a --S.sub.C * or I--S.sub.A --S.sub.C *

whereas materials with the following phase sequences are more difficultto align:

    I--N*--S.sub.C *

Typically, therefore, in order to use a liquid crystal material in thesmectic phase it will involve heating the material to the nematic orisotropic phase and allowing it to cool slowly into an aligned smecticstate. Should this technique be applied to a polymer liquid crystalmaterial then the cooling time is usually very much longer in order toassist the alignment, though very often the alignment is poor.

There is a continued need for new liquid crystal polymers which possessthe properties that allow them to be used in devices including one ormore of the known electro-optic devices.

BRIEF SUMMARY OF THE INVENTION

According to this invention there is provided a material of generalformula I ##STR9## wherein X₁ and X₂ are independently selected from H,CN, COCH₃, CO₂ CH₃ m=at least 5;

X₃ and X₄ are independently selected from formula IA: ##STR10## whereinY is selected from COO, OCO, O, S, CHOH, CHF, CH₂ ;

Q=(CH₂)_(n) wherein one or more non-adjacent methylenes may be replacedby O and n=1-20;

Z is selected from O, S, single covalent bond, COO, OCO;

when Y is CH₂ then n may also be 0; ##STR11## represents any mesogenicgroup; X₃ and X₄ are also independently selected from H, OH, OCOR¹,COOH, CO₂ R¹, (CH₂)_(p) OH, (CH₂)_(p) CO₂ H, --(CH₂)_(p) OR¹ or--(CH₂)_(p) CO₂ R¹ and p=1-20, R¹ =H or C₁₋₁₆ alkyl, when R¹ =C₂₋₁₆alkyl the terminal CH₃ group may be replaced by Br or Cl; provided thatat least one of X₃ and X₄ is selected from formula IA.

The mesogenic group is further defined from general structure II##STR12## A, B, D are selected from the following rings: ##STR13## theabove rings may be substituted with one or more of the followingsubstituents in at least one of the available substitution positions: F,Cl, Br, CH₃, CN, OR, R and NCS where R is given by C₁₋₅ branched orstraight chain alkyl;

Z is selected from CN, F, Cl, NO₂, R, OR, CO₂ R, CF₃, OOCR, NCS, SCN,where R=straight chain or branched chain alkyl and may include from 1-16carbon atoms and including where one or more non-adjacent CH₂ groups maybe substituted by CH(CN), CH(CF₃), CH(Cl), CH(CH₃) in chiral ornon-chiral form;

provided that the total number of rings present is not greater than 4;

W₁ and W₂ are independently selected from COO, OCO, single bond, CH₂CH₂, CH₂ O, OCH₂, O, S, CH═CH, C.tbd.C.

According to a further aspect of this invention liquid crystal polymersmay be synthesised via the cyclization of a suitably functionaliseddiene.

According to a further aspect of this invention, liquid crystal polymersof formula I and variants thereof may be synthesised via the cyclisationof the following general formula III: ##STR14## wherein X, X₁, Y, Y₁ maybe independently selected from H, formula IA, C₁₋₁₆ alkyl, OH, Br, Cl,F, I, CO₂ H provided at least one of X, X₁, Y, Y₁, is selected fromformula IA;

Z and Z₁ are independently selected from H, CHO, COCH₃, CO₂ H, CN, CF₃,F, CO₂ R, Cl where R is a straight chain or branched alkyl group,including chiral chains, containing from 4-12 carbon atoms, or analiphatic or aromatic ring or a suitably functionalised mesogenic sidegroup.

Variants thereof of formula III may also be used to make the compoundsof formula I.

Liquid crystal polymers described by the current invention may be any ofthe known types including homo or co polymers.

Y, in formula IA, may be CHOH and the OH groups used as a point ofattachment for cross-linking agents to form elastomers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the followingdiagrams by way of example only:

FIG. 1: is a synthetic scheme for ester linked LCPs,

FIG. 2: is a synthetic scheme for ether linked LCPs

FIG. 3: is a synthetic scheme for reverse ester linked LCPs

FIG. 4: is a synthetic scheme for hydroxylated spacer group LCPs

FIG. 5: is a synthetic scheme for fluorinated spacer group LCPs

FIG. 6: is a synthetic scheme for direct addition LCPs

FIG. 7: illustrates the chiral separation of compounds described by thecurrent invention

FIG. 8: illustrates poly (1,5-hexadienes) derived from theBaylis-Hillman reaction: double addition

FIG. 9: illustrates poly (1,5-hexadienes) derived from theBaylis-Hillman reaction: single addition

FIG. 10: illustrates a liquid crystal device

FIG. 11 illustrates a pyroelectric device

FIGS. 12 and 13 illustrate front and sectional views respectively of areflective spatial light modulator drawn to different scales, in whichthe materials of the current invention may be incorporated.

DETAILED DESCRIPTION OF THE INVENTION

The following reagents were used in the FIGS. 1-9. Schemes 1-9 relate toFIGS. 1-9 respectively.

    ______________________________________                                        Scheme 1                                                                      ______________________________________                                                a/DCC, DMAP [1]                                                                ##STR15##                                                                    K.sub.2 CO.sub.3, butanone [2]                                                c/polymerisation, Irgacure 184, UV                                    ______________________________________                                    

    ______________________________________                                        Scheme 2                                                                      ______________________________________                                                a/TBAB, KOH, THF [3]                                                           ##STR16##                                                                    K.sub.2 CO.sub.3, butanone [2]                                                c/polymerisation Irgacure 184, UV                                     ______________________________________                                    

    ______________________________________                                        Scheme 3                                                                      ______________________________________                                        a/ (i) n-butyllithium, THF; (ii) CO.sub.2, (iii) H.sub.2 O, H.sup.+  [4]      b/ DCC, DMAP [1]                                                              c/ K.sub.2 CO.sub.3, butanone [2]                                             d/ polymerisation Irgacure 184, UV                                            ______________________________________                                    

    ______________________________________                                                 Scheme 4                                                             ______________________________________                                                 a/ Mg, ether [5]                                                              b/ HCO(CH.sub.2).sub.n OTBMPS [5]                                             c/ TBDPS-Cl, CH.sub.2 Cl.sub.2, Et.sub.3 N [6]                                d/ TBAF, THF, CH.sub.2 Cl.sub.2 [7]                                           e/ HO-mesogen, DEAD [8]                                                       f/ dil HCl [9]                                                                g/ polymerisation Irgacure 184, UV                                   ______________________________________                                    

    ______________________________________                                                Scheme 5                                                              ______________________________________                                                a/ DAST [10]                                                                  b/ polymerisation Irgacure 184, UV                                    ______________________________________                                    

    ______________________________________                                                Scheme 6                                                              ______________________________________                                                a/ OEt, Br(CH.sub.2).sub.n Br [11]                                            b/ BH.sub.3 ether [12]                                                        c/ PDC [13]                                                                   d/ K.sub.2 CO.sub.3, MEK [2]                                                  e/ Ph.sub.3 P = CH.sub.2 Wittig Reaction [13]                                 f/ polymerisation Irgacure 184, UV                                    ______________________________________                                    

    ______________________________________                                        Scheme 7                                                                      ______________________________________                                        a/ chiral resolving agent, e.g. O-methyl mandelic acid (+) or (-) [14]        b/ NaOH, ethanol/water [2]                                                    c/ see scheme 1, steps (a) and (b) and or scheme 2, steps (a) and (b)         d/ polymerisation Irgacure 184, UV                                            ______________________________________                                    

    ______________________________________                                                Scheme 8                                                              ______________________________________                                                a/ 3-Quinuclidinol [10]                                                       b/ see scheme 7 step c/                                                       c/ polymerisation Irgacure 184, UV                                    ______________________________________                                    

    ______________________________________                                        Scheme 9                                                                      ______________________________________                                        a/ PDC [14]                                                                   b/ CH.sub.2 CHX, 3-quinuclidinol where X is selected from CN, COCH.sub.3      or                                                                            CO.sub.2 CH.sub.3 [10]                                                        c/ mesogen-O(CH.sub.2).sub.n COOH/DCC/DMAP or mesogen-O(CH.sub.2).sub.n       -Br/NaH                                                                       d/ polymerisation                                                             ______________________________________                                    

wherein

DCC=dicyclohexylcarbodiimide

DMAP=dimethylaminopyridine

TBAB=tetrabutylammonium bromide

TBMPS-Cl=t-butylmethoxyphenylsilyl chloride

TBDPS=t-butyldiphenylsilane

TBAF=tetra-butylammonium fluoride

PDC=pyridinium dichromate

MEK=methyl ethyl ketone

DAST=diethylaminosulphur trifluoride

The numbers in square brackets [] refer to the following referenceswherein information can be found concerning the relevant syntheticsteps:

[1] Bolton et al Liq Cryst, 12(2), 305, 1992.

[2] Lewthwaite et al, J. Mater Chem, 3(3), 241, 1993.

[3] Takahara et al, Heterocycles, 12, 11, 1979.

[4] Seed et al J. Mater Chem 5(1), 1, 1995.

[5]Vogel's textbook of Practical Organic Chemistry, 5th Edition,Longman, 534-6.

[6] Hanessian et al Can J. Chem, 53, 2975, 1975.

[7] Guindon et al, Tetrahedron Lett, 25, 4717, 1984.

[8] Booth et al, J Mater Chem, 4(5), 747, 1994.

[9] Overman et al, Tetrahedron Lett, 27, 4391, 1986.

[10] Hall et al Supramolecular Science, 1(1), 21, 1994.

[11] Petragnani et al, Synthesis, 521, 1982.

[12] Lane, Chem Rev, 76, 773, 1976.

[13] Villieras et al, Synthesis, 924, 1982.

[14] Caney et al, Tet Letts, 399, 1979.

Reagents used in schemes 1-9 are commercially available from Aldrichexcept for Irgacure 184 which is available from Ciba Geigy. Experimentaldetails given in the references were modified slightly for thepreparation of compounds given in Schemes 1-9. The structure of all ofthe materials were confirmed by a combination of nmr spectroscopyincluding ¹ H nmr (JEOL JNM-GX 270 MH spectrometer), infraredspectroscopy (Perkin-Elmer 457 grating spectrometer and Perkin-Elmer 783grating spectrophotometer) and mass spectrometry (Finnigan-MAT 1020GMSspectrometer). The purity of the compounds was checked by tlc (singlespot) and/or hplc (5 μm, 25×0.46 cm, ODS Microsorb column,methanol, >99%) and that of the polymers by gpc [5 μm, 30×0.75 cm, 2×mixed D PL columns, calibrated using polystyrene standards(Mp=1000-430500), toluene; no monomer present].

The transition temperatures were measured using a Mettler FP5 hot-stageand control unit in conjunction with an Olympus BHSP 753 polarisingmicroscope and by DSC (Perkin Elmer DSC 7). The phase behaviour of thematerials was determined by a combination of optical microscopy (OlympusBH2 polarizing microscope in conjunction with a Mettler FP52 hot-stageand FP5 control unit) and X-ray diffraction (University of Bristol).

A general method for the polymerisation of the dienes is set out below.

A solution of the diene (0.0044 mol) and Irgacure 184 photoinitiator(Ciba-Geigy) (0.09 mmol) in dry dichloromethane (3 ml) was spread over aglass plate (18×25cm) and the solvent evaporated to leave a thin film ofmonomer and photoinitiator. The film was irradiated with UV irradiationfrom a UVA sunlamp (Philips) for 6 h. The resultant polymer was purifiedby precipitation from dry dichloromethane by the addition of methanol,and separated by centrifugation (5000 rpm for 30 min). The purificationprocedure was repeated until the monomer precursor had been completelyremoved (tlc silica gel, dichloromethane). The polymer was thendissolved in dry dichloromethane (5 ml) and the resulting solution addeddropwise to petroleum fraction (bp 40-60° C.) with vigorous stirring.The white precipitate that was produced was recovered by filtration andwashed with two further quantities of petroleum fraction (bp 40-60° C.)(2×50 cm³) and then redissolved in dry dichloromethane (5 cm³). Thissolution was then passed through a 0.5 μm membrane filter and thesolvent removed to leave the polymer as a glass. It was then dried at50° C. in vacuo for 6 h.

The following experimental details, by way of example only, illustratehow the cyclic poly (1,5-hexadienes) were made. ##STR17##

1,5-Hexadiene,3,4-diol (0.013 mol), 11- bromoundecanoic acid (0.026mol), dicyclohexylcarbodiimide (DCC) (0.030 mol) andN,N-dimethylaminopyridine (DMAP) (0.20 g) in dry dichloromethane (75cm³) were stirred together for 6 h at room temperature. Removal ofdicyclohexyurea by filtration, followed by removal of solvent in vacuoleft a yellow oil which solidified on standing. Recrystallization frombutanone gave compound 1 as a white solid (75%), mp 43-45° C. ##STR18##

Dibromodiene (1) (0.0050 mol ), 4cyano4'-hydroxybiphenyl (0.01 mol) andpotassium carbonate (3.0 g) were refluxed together in dry butanone (60cm³) for 24 h. Excess potassium carbonate was removed by filtrationfollowed by removal of solvent in vacuo to leave a white solid which waspurified using column chromatography (silica gel), with ethyl acetate asthe eluent. Recrystallization from acetonitrile gave compound 2 as awhite powder (89%), mp 72-74° C. ##STR19##

Monomer (2) (0.0043 mol) and Irgacure 184 photoinitiator (Ciba-Geigy)(0.022 mmol) were dissolved in dry dichloromethane (3.0 cm³) and theresultant solution was spread evenly on a borosilicate glass plate(25×18 cm²).The solvent was evaporated in air to leave a monomer film. Asimilar glass plate was placed over the monomer film and the two plateswere squeezed together to produce a very thin film between the plates.The monomer "sandwich" was then irradiated beneath a Philips UVA (70 w.)sunlamp for 30 min. The resultant polymer was removed from the plates,suspended in methanol and centrifuged (10 min. at 11,000 rpm). Thecentrifugation was repeated a further three times and then the polymerwas dissolved in dry dichloromethane (10 cm³). The resultant solutionwas passed through 0.5 μm membrane filter. Removal of solvent gavepolymer 3 as a cream coloured solid (55%). ##STR20##

Tetrabutylammonium bromide (TBAB) (0.0012 mol) was added to finelyground potassium hydroxide(0.010 mol) in dry tetrahydrofuran (30cm³).3-Hydroxy-1,5-hexadiene (0.010 mol) and 1,8-dibromooctane (0.012 mol)were added and the mixture was stirred at room temperature for 18 h Thesolvent was removed in vacuo and the liquid residue dissolved indichloromethane (50 cm³). The resultant solution was washed in water(2×20 cm³), then hydrochloric acid (1M) (20 cm³), water (20 cm³), andthen finally dried over magnesium sulphate. Removal of solvent in vacuoleft a yellow oil which was purified by column chromatography (silicagel) using dichloromethane as the eluent to leave compound 4 as a clear,colourless oil (58%). ##STR21##

4-Cyano,4-hydroxybiphenyl (0.0034 mol), compound 4 (0.0040 mol) andpotassium carbonate (5 g) were refluxed together in dry butanone (60cm³) for 18 h. Filtration to remove excess potassium carbonate followedby removal of solvent in vacuo left a white solid which was purified bycolumn chromatography (silica gel) using dichloromethane as the eluent.Recrystallisation from acetonitrile gave compound 5 as a white powder(84%) mp 91-93° C. ##STR22##

Monomer 5 (0.00124 mol) and Irgacure 184 photoinitiator (Ciba-Geigy)(0.0062 mmol) were dissolved in dichloromethane (3 cm³) and the solutionwas spread evenly on a borosilicate glass plate (25×18 cm²). The solventwas removed in air to leave a film of monomer/initiator. A similar glassplate was placed over the monomer and the two plates were squeezedtogether to produce a very thin monomer film. The monomer "sandwich" wasirradiated beneath a Philips UVA (70 w) sunlamp for 30 min. Theresultant polymer was removed and suspended in methanol and purified bycentrifugation (10 min. at 11,000 r.p.m.). This was repeated a furtherthree times then the recovered polymer was dissolved in drydichloromethane (10 cm³). The solution was passed through a 0.5μmembrane filter. Removal of solvent gave polymer 6 as a white solid.(55%).

The following compounds (1-7) are illustrative examples which have beensynthesised according to the present invention:

Homo-Poly(1,5-hexadienes) mono-mesogenic side chain

    __________________________________________________________________________     ##STR23##                                                                                                         transition temperatures/C.               __________________________________________________________________________     ##STR24##                           g 52 I                                    ##STR25##                           g 30.5 SA 58.5 I                          ##STR26##                           g 31.0 SA 221.0 I                         ##STR27##                           g -22.5 SA 30.0 I                         ##STR28##                           g 35.0 SA 80.0 I                          ##STR29##                           g 5.5 SA 42.3 I                           ##STR30##                           g 21.0 Sc* 78.5 I                        __________________________________________________________________________

GPC data for the above compounds 1-7 is as follows:

    __________________________________________________________________________    Compound                                                                      1        2    3    4    5    6    7                                           __________________________________________________________________________    M.sub.W                                                                           7970 9070 18540                                                                              14400                                                                              9080 9820 17300                                       M.sub.n                                                                           5140 5650 11380                                                                              8140 5570 6200 8480                                        M.sub.W /M.sub.n                                                                  1.5  1.6  1.6  1.8  1.6  1.6  2.0                                         DP  12   12   20   15   11   11   12                                          __________________________________________________________________________

Further examples are 8, 8a and 8b:

    __________________________________________________________________________     ##STR31##                                                                    R =                                                                           __________________________________________________________________________     ##STR32##                              Mw = 9430  Mn = 7580  Mw/Mn = 1.3                                              DP = 19                               ##STR33##                              Mw = 9672  Mn = 7440  Mw/Mn = 1.3                                              DP = 13                               ##STR34##                              Mw = 12192  Mn = 7620  Mw/Mn =                                                1.6  DP = 11                          __________________________________________________________________________

Di-mesogenic side chain (compounds 9-12)

    __________________________________________________________________________     ##STR35##                                                                    R                                    transition temperatures/C.               __________________________________________________________________________     ##STR36##                           g 53.5 SA 80.0 I                          ##STR37##                           g 28.0 SA 86.0 I                          ##STR38##                           g 19.0 SA 132.5 I                         ##STR39##                           g 8.0 Sc* 95.0 SA 110.0                  __________________________________________________________________________                                         I                                    

    ______________________________________                                               Compound                                                                      9     10          11      12                                           ______________________________________                                        M.sub.W  7990    13600       27400 15500                                      M.sub.n  5170    7360        13800 10900                                      M.sub.W /M.sub.n                                                                       1.5     1.8         1.9   1.4                                        DP       7       10          18    9                                          ______________________________________                                    

The following illustrates an example of a co-polymer made according tothe present invention, co-poly(1,5-hexadienes): ##STR40##

R₁ =(CH₂)₁₀ Br

Transition Temperatures/°C. g 64.5 S_(A) 157.5 I.

GPC data: M_(w) =15900, M_(n) =8960, M_(w) /M_(n) =1.8, DP=14

An example of the use of a material and device embodying the presentinvention will now be described with reference to FIG. 10.

The liquid crystal device consists of two transparent plates, 1 and 2,for example made from glass. These plates are coated on their internalface with transparent conducting electrodes 3 and 4. An alignment layer5,6 is introduced onto the internal faces of the cell so that a planarorientation of the molecules making up the liquid crystalline materialwill be approximately parallel to the glass plates 1 and 2. This is doneby coating the glass plates 1,2 complete with conducting electrodes sothat the intersections between each column and row form an x, y matrixof addressable elements or pixels. For some types of display thealignment directions are orthogonal. Prior to the construction of thecell the layers 5,6 are rubbed with a roller covered in cloth (forexample made from velvet) in a given direction, the rubbing directionsbeing arranged parallel (same or opposite direction) upon constructionof the cell. A spacer 7 e.g. of polymethyl methacrylate separates theglass plates 1 and 2 to a suitable distance e.g. 2 microns. Liquidcrystal material 8 is introduced between glass plates 1,2 by filling thespace in between them. This may be done by flow filling the cell usingstandard techniques. The spacer 7 is sealed with an adhesive 9 in avacuum using an existing technique. Polarisers 10, 11 may be arranged infront of and behind the cell.

Alignment layers may be introduced onto one or more of the cell walls byone or more of the standard surface treatment techniques such asrubbing, oblique evaporation or as described above by the use of polymeraligning layers.

In alternative embodiments the substrates with the aligning layers onthem are heated and sheared to induce alignment, alternatively thesubstrates with the aligning layers are thermally annealed above theglass transition temperature and below the liquid crystal to isotropicphase transition in combination with an applied field. Furthercombinations may involve a combination of these aligning techniques.With some of these combinations an alignment layer may not be necessary.

The device may operate in a transmissive or reflective mode. In theformer, light passing through the device, e.g. from a tungsten bulb, isselectively transmitted or blocked to form the desired display. In thereflective mode a mirror, or diffuse reflector, (12) is placed behindthe second polariser 11 to reflect ambient light back through the celland two polarisers. By making the mirror partly reflecting the devicemay be operated both in a transmissive and reflective mode.

The alignment layers 5,6 have two functions, one to align contactingliquid crystal molecules in a preferred direction and the other to givea tilt to these molecules--a so called surface tilt--of a few degreestypically around 4° or 5°. The alignment layers 5,6 may be formed byplacing a few drops of the polyimide on to the cell wall and spinningthe wall until a uniform thickness is obtained. The polyimide is thencured by heating to a predetermined temperature for a predetermined timefollowed by unidirectional rubbing with a roller coated with a nyloncloth.

In an alternative embodiment a single polariser and dye material may becombined.

The liquid crystal material 8 when introduced into the cell may consistof liquid crystal polymer or consist of liquid crystal monomers and aphotoinitiator. It may also contain a reagent which will limit themolecular weight of the polymer for example a chain transfer reagent andit may also include a thermal initiator.

The monomer material may be aligned before polymerisation using standardtechniques, for example by heating up to and cooling from the isotropicphase or from a liquid crystal phase such as a nematic or chiral nematicphase. It is also possible that the liquid crystal polymer may bealigned by one or more techniques including the use of surface forces,shear alignment or field alignment.

It is possible that following polymerisation there may still be someamount of monomer material remaining. This may be unreacted monomer orlow molar mass additives which do not bear polymerisable groups.

Polymerisation may be carried out by using any of the known techniques.For example the monomer material plus initiator may be exposed to UVlight, heat may also be applied to permit polymerisation within a givenphase of the monomer and/or polymer.

Alternatively the polymerisation process may take place in the presenceof heat and a thermal initiator. However if this technique is used itmay be preferable if it is carried out at a temperature whichcorresponds to a liquid crystal phase of the monomer material.

In-situ polymerisations are described in UK Patent Application GB9420632.3 which also describes the use of chain transfer reagents tocontrol molecular weight of liquid crystal polymers. As mentioned abovethere may also be a chain transfer reagent present in the mixture of thecurrent invention. GB 9514970.4 describes in-situ polymerisations in thepresence of a cationic photoinitiator.

Many of the compounds described by formula I and mixtures includingcompounds of formula I show liquid crystalline behaviour and are thususefully employed in liquid crystal devices. Example of such devicesinclude optical and electro-optical devices, magneto-optical devices anddevices providing responses to stimuli such as temperature changes andtotal or partial pressure changes. The compounds of formula I may alsobe included in a mixture, where the mixture comprises at least twocompounds. Typical mixtures include mixtures consisting of compounds offormula I and also mixtures comprising at least one compound of formulaI and at least one compound not of formula I

Materials have been proposed for laser addressed applications in whichlaser beams are used to scan across the surface of the material or leavea written impression thereon. For various reasons many of thesematerials have consisted of organic materials which are at leastpartially transparent in the visible region. The technique relies uponlocalised absorption of laser energy which causes localised heating andin turn alters the optical properties of the otherwise transparentmaterial in the region of contact with the laser beam. Thus as the beamtraverses the material, a written impression of its path is left behind.One of the most important of these applications is in laser addressedoptical storage devices, and in laser addressed projection displays inwhich light is directed through a cell containing the material and isprojected onto a screen. Such devices have been described by Khan Appl.Phys. Lett vol. 22, p111, 1973; and by Harold and Steele in Proceedingsof Euro display 84, pages 29-31, September 1984, Paris, France, in whichthe material in the device was a smectic liquid crystal material.Devices which use a liquid crystal material as the optical storagemedium are an important class of such devices. The use of semiconductorlasers, especially Ga_(x) Al_(1-x) As lasers where x is from 0 to 1, andis preferably 1, has proven popular in the above applications becausethey can provide laser energy at a range of wavelengths in the nearinfra-red which cannot be seen and thus cannot interfere with the visualdisplay, and yet can provide a useful source of well-defined, intenseheat energy. Gallium arsenide lasers provide laser light at wavelengthsof about 850nm, and are useful for the above applications. Withincreasing Al content (x<1), the laser wavelength may be reduced down toabout 750 nm. The storage density can be increased by using a laser ofshorter wavelength.

The compounds of the present invention may be suitable as opticalstorage media and may be combined with dyes for use in laser addressedsystems, for example in optical recording media.

The smectic and/or nematic properties of the materials described by thecurrent invention may be exploited. For example the materials of thepresent invention nay be used in ferroelectric mixtures and devices.

The compounds of the present invention may also be used in pyroelectricdevices for example detectors, steering arrays and vidicon cameras.

FIG. 11 illustrates a simple pyroelectric detector in which thematerials of the present invention may be incorporated.

A pyroelectric detector consists of electrode plates 1,2 at least one ofwhich may be pixellated. In operation the detector is exposed toradiation R, for example infrared radiation, which is absorbed by theelectrode 1. This results in a rise in temperature which is transmittedto a layer of pyroelectric material 3 by conduction, The change intemperature results in a thermal expansion and a charge is generated.This change in charge is usually small when compared with the chargeoutput due to the change in the spontaneous polarisation, Ps with achange in temperature; this constitutes the primary pyroelectric effect.A change in charge results in a change in potential difference betweenthe electrodes. The charge on each pixel may be read out and theresulting signal is used to modulate scanning circuits in, for example,a video monitor and for a visual image of the infra red scans.

The selective reflective properties of the materials described by thecurrent invention may also allow for materials of the current inventionto be used in inks and paints and they may therefore be useful inanti-counterfeiting operation. They may also be used in so-calledsecurity inks. Other applications include thermal control management,for example the materials may be included in a coating which may beapplied to one or more windows in order to reflect infra-red radiation.

As shown in FIGS. 12 and 13 a spatial light modulator comprises a liquidcrystal cell 1 formed by typically two glass walls 2 and 3 and 0.1-10 μme.g. 2.5 μm thick spacer 4. The inner faces of the walls carry thintransparent indium tin oxide electrodes 5,6 connected to a variablevoltage source 7. On top of the electrodes 5,6 are surface alignmentlayers 8,9 e.g. of rubbed polyimide described in or detail later. Otheralignment techniques are also suitable e.g. non-rubbing techniques suchas evaporation of SiO₂. A layer 10 of liquid crystal material iscontained between the walls 2,3 and spacer 4. In front of the cell 1 isa linear polariser 11; behind the cell 1 is a quarter plate 12 (this maybe optional) and a mirror 13. An example of a linear polariser is apolarising beam splitter (not illustrated here).

There are a variety of electroclinic devices in which the compounds ofthe present invention may be incorporated. For example in the abovedescription of FIGS. 12 and 13, active black plane driving may beutilised. One of the walls forming the cell may be formed from a siliconsubstrate e.g. a wafer which possesses circuitry for driving pixels.

For many of these devices there exists an optimum thickness for the cellwhich is related to the birefringence (Δn) given by: ##EQU1## whereinλ=wavelength of operation

Δn=birefringence of liquid crystalline material

m=integer.

Some suitable methods for driving electroclinic devices described by thepresent invention may be found in UK patent application GB-2 247 972 A.

The mode of operation of the devices described by the current inventionincludes either amplitude modulation or phase modulation. Similarlydevices may be used in reflectance or transmissive mode.

The materials of this aspect of the invention may be used in many of theknown forms of liquid crystal display devices, for example chiralsmectic electro-optic devices. Such a device may comprise a layer ofliquid crystal material contained between two spaced cell walls bearingelectrode structures and surface treated to align liquid crystalmaterial molecules. The liquid crystal mixtures may have manyapplications including in ferroelectric, thermochromic and electroclinicdevices.

The compounds of the present invention may be mixed with each other toform useful liquid crystal mixtures, hey may also be used with otherliquid crystal polymers or low molar mass non-polymer liquid crystalmaterials.

Suitable devices in which the materials of the current invention may beincorporated include beam steerers, shutters, modulators andpyroelectric and piezoelectric sensors.

The materials of the present invention may also be useful as dopants inferroelectric liquid crystal devices, which may be multiplexed, or theymay be used in active backplane ferroelectric liquid crystal systems.The materials of the present invention may also be useful as hostmaterials. The materials of the present invention may be included inmixtures which also contain one or more dopants.

Compounds of formula I may be mixed with a wide range of hosts, forexample smectic hosts to form a useful liquid crystal composition. Suchcompositions can have a range of Ps values. Compounds of formula I maybe mixed with one or more of the types of hosts VIII-XIII. Thesedifferent types of hosts may be mixed together to which the compound ofgeneral formula I may also be added.

Typical hosts include:

The compounds described in PCTIGB86/00040, e.g. of formula VIII##STR41## where R₁ and R₂ are independently C₃ -C₁₂ alkyl or alkoxy.

The fluoro-terphenyls described in EPA 84304894.3 and GBA 8725928, e.g.of formula IX ##STR42## where R₁ and R₂ are independently C₃ -C₁₂ alkylor alkoxy, x is 1 and F may be on any of the available substitutionpositions on the phenyl ring specified.

The difluoro-terphenyls described in GBA 8905422.5, e.g. of formula X##STR43## where R₁ and R₂ are independently C₃ -C₁₂ alkyl or alkoxy.

The phenyl-pyrmidines described in WO 86/00087, e.g. of formula XI.##STR44## including those compounds where R₁ is C₃ -C₁₂ alkyl and R₂ isgiven by the general formula (CH₂)_(n) CHXCH₂ CH₃, where n is 1 to 5 andX is CN or Cl.

The compounds described by R. Eidenschink et al in Cyclohexanederivativemit Getilteneten Smektischen Phasen at the 16^(th) Freiberg LiquidCrystal Conference, Freiberg, Germany, p8. Available from E Merck Ltd,Germany, e.g. of formula XI. ##STR45## including those compounds whereR₁ and R₂ are independently C₁ -C₁₅ alkyl.

The difluoro-phenyl pyrimidines described at the 2^(nd) InternationalSymposium on Ferroelectric Liquid Crystals, Goteborg, Sweden, June 1989by Reiffenrath et al, e.g. of formula XIII ##STR46## including thosecompounds where R₁ and R₂ are independently C₃ -C₉ alkyl.

The materials of the current invention may also be useful inthermochromic devices, for example those devices described by D. G,McDonnell in Thermochromic Liquid Crystals, Critical Reports on AppliedChemistry, vol. 22, edited by G. W. Gray, 1987 pp120-44 and referencestherein.

We claim:
 1. A material of general formula I ##STR47## wherein X₁ and X₂are independently selected from H, CN, COCH₃, CO₂ CH₃ m=at least 5;X₃and X₄ are independently selected from formula IA: ##STR48## wherein Yis selected from COO, OCO, O, S, CHOH, CHF, CH₂ ; Q=(CH₂)_(n) whereinone or more non-adjacent methylenes may be replaced by O and n=1-20; Zis selected from O, S, single covalent bond, COO, OCO; when Y is CH₂then n may also be 0; ##STR49## represents any mesogenic group; X₃ andX₄ are also independently selected from H, OH, OCOR¹, COOH, CO₂ R¹,(CH₂)_(p) OH, (CH₂)_(p) CO₂ H, --(Ch₂)_(p) OR¹ or --(Ch₂)_(p) CO₂ R¹ andp=1-20, R¹ =H or C₁₋₁₆ alkyl; when R¹ =C₂₋₁₆ alkyl the terminal CH₃group may be replaced by Br or Cl; provided that at least one of X₃ andX⁴ is selected from formula IA.
 2. A material according to claim 1wherein the mesogenic group is given by the general structure II##STR50## A, B, D are selected from the following rings: ##STR51## theabove rings may be substituted with one or more of the followingsubstituents in at least one of the available substitution positions: F,Cl, Br, CH₃, CN, OR, R and NCS where R is given by C₁₋₅ branched orstraight chain alkyl;Z is selected from CN, F, Cl, NO₂, R, OR, CO₂ R,CF₃, OOCR, NCS, SCN, where R=straight chain or branched chain alkyl andmay include from 1-16 carbon atoms and including where one or morenon-adjacent CH₂ groups may be substituted by CH(CN), CH(CF₃), CH(Cl),CH(CH₃) in chiral or non-chiral form; provided that the total number ofrings present is not greater than 4; W₁ and W₂ are independentlyselected from COO, OCO, single bond, CH₂ CH₂, CH₂ O, OCH₂, O, S, CH═CH,C.tbd.C.
 3. A liquid crystal mixture comprising at least one of thecompounds of claim
 1. 4. A ferroelectric mixture comprising at least oneof the compounds of formula I.
 5. A cholesteric liquid crystal mixturecomprising at least one of the compounds of claim
 1. 6. A liquid crystalmixture containing any of the compounds of claim 1 and a material of thefollowing general formula: ##STR52## where R₁ and R₂ are independentlyC₃ -C₁₂ alkyl or alkoxy.
 7. A liquid crystal mixture containing any ofthe compounds of claim 1 and a material of the following generalformula: ##STR53## where R₁ and R₂ are independently C₃ -C₁₂ alkyl oralkoxy, x is 1 and F may be on any of the available substitutionpositions on the phenyl ring specified.
 8. A liquid crystal mixturecontaining any of the compounds of claim 1 and a material of thefollowing general formula: ##STR54## where R₁ and R₂ are independentlyC₃ -C₁₂ alkyl or alkoxy.
 9. A liquid crystal mixture containing any ofthe compounds of claim 1 and a material of the following generalformula: ##STR55## including those compounds where R₁ is C₃ -C₁₂ alkyland R₂ is given by the general formula (CH₂)_(n) CHXCH₂ CH₃, where n is1 to 5 and X is CN or Cl.
 10. A liquid crystal mixture containing any ofthe compounds of claim 1 and a material of the following generalformula: ##STR56## including those compounds where R₁ and R₂ areindependently C₁ -C₁₅ alkyl.
 11. A liquid crystal mixture containing anyof the compounds of claim 1 and a material of the following generalformula: ##STR57## including those compounds where R₁ and R₂ areindependently C₃ -C₉ alkyl.
 12. A device comprising two spaced cellwalls each bearing electrode structures and treated on at least onefacing surface with an alignment layer, a layer of a liquid crystalmaterial enclosed between the cell walls, characterised in that itincorporates the liquid crystal mixture as claimed in claim
 4. 13. Apyroelectric device comprising two spaced electrodes and a layer of aliquid crystal material enclosed between the electrodes, characterisedin that it incorporates the liquid crystal mixture as claimed in claim3.
 14. A piezoelectric device comprising two spaced electrodes and alayer of a liquid crystal material enclosed between the electrodes,characterised in that it incorporates the liquid crystal mixture asclaimed in claim
 3. 15. A liquid crystal electro-optical display devicecharacterised in that it incorporates a mixture as claimed in claim 4.16. An optical recording medium comprising a recording layer whichcomprises one or more compounds of claim 1 and a dye material.